The invention relates to a method and system for providing a Mobile IP key, in particular for WiMax networks.
The internet with the TCP/IP protocol offers a platform for the development of higher-level protocols for the mobile domain. Because the internet protocols are widely established, a large user community can be opened up by corresponding protocol extensions for mobile environments. However, the traditional internet protocols were originally not designed for mobile applications. In the packet-switching operation of the traditional internet the packets are exchanged between stationary computers which neither change their network address nor roam between different subnetworks. In radio networks including mobile terminals or computers, mobile computers MS (Mobile Station) are frequently integrated into different networks. With the aid of a corresponding server the DHCP (Dynamic Host Configuration Protocol) enables an IP address and further configuration parameters to be assigned dynamically to a computer in a network. A computer that is integrated into a network is automatically assigned a free IP address by the DHCP protocol. A mobile computer with DHCP installed only needs to come into range of a local area network that supports configuration via the DHCP protocol. With the DHCP protocol, dynamic address assignment is possible, i.e. a free IP address is automatically allocated for a specific period of time. After this period of time has elapsed, either the request must be repeated by the mobile computer MS or the IP address can be assigned elsewhere.
With DHCP, a mobile computer MS can be integrated into a network without manual configuration. The only prerequisite is that a DHCP server is available. A mobile computer MS can thus use services of the local area network and for example use centrally stored files. If, on the other hand, a mobile computer MS itself provides services, a potential service user cannot locate the mobile computer MS because its IP address changes in each network in which the mobile computer is integrated. The same happens if an IP address changes during an existing TCP connection. This leads to the connection being aborted. With Mobile IP, a mobile computer MS is for that reason assigned an IP address which it also retains in a different network. With a traditional IP network change it is necessary to adjust the IP address settings accordingly. A constant adjustment of IP configuration mechanisms and the traditional automatic configuration mechanisms will interrupt the existing connection in the event of a change in the IP address. The MIP protocol (RFC2002, RFC2977, RFC3344, RFC3846, RFC3957, RFC3775, RFC3776, RFC4285) supports the mobility of mobile terminals MS. With the traditional IP protocols, the mobile terminal MS must adjust its IP address every time it changes the IP subnetwork in order to ensure that the data packets addressed to the mobile terminal MS are correctly routed. In order to maintain an existing TCP connection, the mobile terminal MS must retain its IP address, since a change of address leads to an interruption of the connection. The MIP protocol allows a transparent connection between the two addresses, which is to say a permanent home address and a second, temporary, care-of address. The care-of address is the IP address under which the mobile terminal MS can be reached at the present time.
A home agent HA is a representative of the mobile terminal MS while the mobile terminal MS is not resident in the original home network. The home agent is kept constantly informed about the current whereabouts of the mobile computer MS. The home agent HA usually constitutes a component of a router in the home network of the mobile terminal. When the mobile terminal MS is located outside of the home network, the home agent HA provides a function that enables the mobile terminal MS to log in. The home agent HA then forwards the data packets addressed to the mobile terminal MS into the current subnetwork of the mobile terminal MS.
A foreign agent FA is located in the subnetwork in which the mobile terminal MS is on the move. The foreign agent FA forwards incoming data packets to the mobile terminal MS or, as the case may be, to the mobile computer MS. The foreign agent FA is located in a foreign network which is referred to as a visited network. The foreign agent FA likewise usually constitutes a component of a router. The foreign agent FA routes all the administrative mobile data packets between the mobile terminal MS and its home agent HA. The foreign agent FA unpacks the tunneled IP data packets sent by the home agent HA and forwards their data to the mobile terminal MS.
The home address of the mobile terminal MS is a permanent address at which the mobile terminal MS can be reached at all times. The home address has the same address prefix as the home agent HA. The care-of address is the IP address which the mobile terminal MS uses in the visited network.
The home agent HA maintains what is called a Mobility Binding Table (MBT). The entries in said table serve to associate the two addresses, i.e. the home address and the care-of address, of a mobile terminal MS with each other and to redirect the data packets accordingly.
The MBT table contains entries relating to the home address, the care-of address and a value specifying the period of time during which this assignment is valid (lifetime).
FIG. 1 shows an example of a mobility binding table MBT according to the related art.
The foreign agent FA includes a visitor list (VL) containing information about the mobile terminals MS that currently reside in the IP network of the foreign agent FA.
FIG. 2 shows an example of a visitor list of said kind according to the related art.
In order for a mobile computer MS to be able to be integrated into a network it must first discover whether it is located in its home network or in a visited network. In addition the mobile terminal MS must find out which computer is in the subnetwork of the home or foreign agent. This information is determined by what is termed agent discovery.
By the subsequent registration the mobile terminal MS can communicate its current location to its home agent HA. For that purpose the mobile computer or mobile terminal MS sends the current care-of address to the home agent. In order to register, the mobile computer MS sends a registration request to the home agent. The home agent HA enters the care-of address in its list and responds with a registration reply. Therein lies a security problem, however. Since in principle any computer can send a registration request to a home agent HA, a home agent HA could easily be deceived into believing a computer had moved into a different network. In this way a foreign computer could intercept all the data packets of a mobile computer or mobile terminal MS without a sender learning about it. In order to prevent this, the mobile computer MS and the home agent HA have a shared secret key. When a mobile computer MS returns to its home network, it deregisters from the home agent HA, since the mobile computer MS can now receive all the data packets itself. A mobile radio network must have a plurality of security characteristics, including the following: Information may only be made accessible to desired communication partners, i.e. undesirable eavesdroppers must not obtain any access to transmitted data. The mobile radio network must therefore have the characteristic of confidentiality. In addition authenticity must be established. Authenticity allows a communication partner to determine beyond doubt whether a communication has actually been set up to a desired communication partner or whether a foreign party is masquerading as a communication partner. Authentications can be performed per message or per connection. If authentication is performed on the basis of connections, the communication partner is identified only once at the start of a session. For the remainder of the session it is then assumed that subsequent messages continue to originate from the corresponding sender. Even when the identity of a communication partner is established, i.e. the communication partner has been authenticated, the situation can occur that said communication partner is not allowed to access all resources or is not allowed to use all services via the network. In this case a corresponding authorization is conditional on a related authentication of the communication partner.
In mobile data networks messages have to travel relatively long distances over air interfaces and consequently are easily accessible to potential attackers. Security aspects therefore play a special role in mobile and wireless data networks. Encryption techniques represent a significant way to increase security in data networks. As a result of the encryption it is possible to transmit data over insecure communication paths, over air interfaces for example, without unauthorized third parties gaining access to the data. For encryption purposes the data, i.e. what is referred to as plaintext, is transformed with the aid of an encryption algorithm into cipher text. The encrypted text can be transported over the insecure data transmission channel and subsequently decrypted or deciphered.
As a promising wireless access technology, WiMax (Worldwide Interoperability for Microwave Access) is being proposed as a new standard that uses IEEE 802.16 for the wireless transmission. The aim with WiMax is to provide coverage over a range of up to 50 km at data rates of over 100 Mbits per second by transmitter stations.
FIG. 3 shows a reference model for a WiMax radio network. A mobile terminal MS is located in the area of an access serving network (ASN). The access serving network ASN is connected to a home network HCSN (Home Connectivity Service Network) via at least one visited network (Visited Connectivity Service Network VCSN) or intermediate network. The different networks are connected to one another via interfaces or reference points R. The home agent HA of the mobile station MS is located in the home network (HCSN) or in one of the visited networks (VCSN).
WiMax supports two implementation variants of Mobile IP, namely what is termed a Client MIP (CMIP), in which the mobile station MS itself implements the MIP client function, and a Proxy MIP (PMIP), in which the MIP client function is implemented by the WiMax access serving network ASN. The functionality provided for that purpose in the ASN is referred to as a Proxy Mobile Node (PMN) or as a PMIP client. This enables MIP to be used also with mobile stations MS which themselves do not support MIP.
FIG. 4 shows the connection setup for Proxy MIP (PMIP) when the home agent HA is located in the visited network VCSN, according to the related art.
Following setup of a wireless connection between the mobile terminal MS and a base station BS, an access authentication is performed first. The authentication, authorization and accounting (AAA) function is performed by what are termed AAA servers. Authentication messages are exchanged between the mobile terminal MS and the AAA server of the home network (HAAA); the address of the home agent HA and an authentication key are obtained by said messages. The authentication server in the home network contains the profile data of the subscriber. The AAA server receives an authentication request message containing a subscriber identity of the mobile terminal. Following successful access authentication the AAA server generates an MSK key (MSK: Master Session Key) in order to protect the data transmission link between the mobile terminal MS and the base station BS of the access serving network ASN. Said MSK key is transmitted by the AAA server of the home network via the intermediate network CSN to the access serving network ASN.
Following the access authentication, the DHCP proxy server is configured in the access serving network ASN, as can be seen in FIG. 4. If the IP address and host configuration are already contained in the AAA reply message, all of the information is downloaded into the DHCP proxy server.
Following successful authentication and authorization the mobile station or mobile terminal MS sends a DHCP discovery message and an IP address is assigned.
When a mobile terminal MS is integrated into a network, the mobile terminal MS must possibly be able to ascertain whether it is located in a home or a visited network. The mobile terminal MS must also find out which computer is the home or, as the case may be, foreign agent in the respective network. This information is determined by what is termed agent discovery. There are two types of agent discovery, namely what is termed agent advertisement and agent solicitation.
With agent advertisement, the agents, i.e. the home or foreign agents, periodically send broadcast messages to all computers or mobile terminals of the subnetwork. Any computer that intercepts the broadcast messages in a specific period of time can thus identify the agents in the respective subnetwork.
When a mobile terminal MS is reactivated it is generally not practical to wait for the next agent advertisement. The mobile terminal MS has to know immediately in which subnetwork it is currently located. With so-called agent solicitation, the mobile terminal MS therefore sends a request to all computers of the respective subnetwork to perform an agent advertisement. By agent solicitation the mobile terminal MS can force the agents to identify themselves immediately, thereby considerably shortening the waiting time. Agent solicitation is also performed if an agent advertisement fails to materialize, for example in the case of packet loss or network change. With the aid of agent discovery a mobile terminal MS can also ascertain whether it is located in its home network or in a visited network. The mobile terminal MS recognizes its home agent HA on the basis of the packet information contained within an agent advertisement message. If the mobile terminal MS receives message packets from a visited network, it can additionally establish whether its location has changed since the last advertisement. If the mobile terminal MS receives no advertisement message, it initially assumes that it is located in the home network and that the home agent HA has a problem. The mobile terminal MS then attempts to contact the router of the network in order to confirm this assumption. If the mobile terminal MS is not located in its home network, it thereupon tries to reach a DHCP server and obtain an address of the subnetwork. If this is successful, the mobile terminal MS uses this address as what is called a colocated care-of address and makes contact with the home agent HA. The colocated care-of address is an address that is assigned to the mobile terminal MS in the visited network and also transmitted to the home agent HA.
A distinction is drawn between network-based mobility management (PMIP) and terminal-based mobility management (CMIP). With terminal-based mobility management CMIP, the terminal supports Mobile IP (MIP).
FIG. 4 shows the connection setup in the case of traditional network-based mobility management (PMIP), while FIG. 5 represents the connection setup in the case of traditional terminal-based mobility management (CMIP).
During the setting up of a connection between the mobile terminal MS and the network, the authentication server of the home network (H-AAA) sends an authentication acknowledgement message (SUCCESS) following successful authentication of the subscriber. The authentication acknowledgement message reports to the authentication client that the authentication of the subscriber has been successfully completed.
With Proxy MIP or network-based mobility management (PMIP), the mobile terminal does not support Mobile IP or the corresponding MIP software is not activated in the mobile terminal MS.
In contrast, with Client MIP (CMIP) or terminal-based mobility management, Mobile IP is supported by the respective terminal or mobile station MS.
In the case of Proxy MIP the mobile terminal MS only recognizes an IP address assigned by the DHCP server. The care-of address of the mobile terminal MS is not known to the mobile terminal, but is known to the PMIP client the foreign agent FA and the home agent HA. In contrast, in the case of Client MIP, the mobile terminal MS recognizes both of its IP addresses, i.e. both the home address and the care-of address.
As can be seen in FIGS. 4, 5, an MIP registration takes place following the IP address assignment. With the MIP registration, the home agent HA is informed about the current location of the mobile terminal MS. For its registration the mobile terminal MS or the corresponding PMIP client sends a registration request containing the current care-of address to a home agent HA. The home agent HA enters the care-of address in a list administered by it and responds with a registration reply. Since in principle any computer can send a registration request to a home agent HA, a home agent HA could easily be deceived into believing a computer or a mobile terminal MS had moved into a different network. In order to prevent this, both the mobile terminal MS and the home agent HA have a shared secret key, i.e. what is referred to as a Mobile IP key (MIP-KEY).
In the case of Proxy MIP (PMIP) the registration request (MIPRRQ) is transmitted via a foreign agent FA to the home agent HA by a PMIP client within the access serving network ASN. The home agent HA has a key for the subscriber assigned by the associated authentication server H-AAA and transmits said key with the MIP registration reply (MIP Registration Reply), as shown in FIG. 4.
In terminal-based mobility management (CMIP) the registration request message (MIPRRQ) is routed directly from the mobile terminal MS via the foreign agent FA to the home agent HA, as shown in FIG. 5.
As can be seen from FIGS. 4, 5, the same Mobile IP key (MIP Key) is thus provided in the case of PMIP and CMIP during the access authentication by the authentication server for the purpose of encrypting Mobile IP signaling messages. The Mobile IP key serves for encrypting Mobile IP signaling messages between the Mobile IP client and the Mobile IP home agent HA.
For the CMIP case shown in FIG. 5, the authenticator therefore receives a Mobile IP key that it does not need at all. The authenticator is typically located in a gateway node of the access serving network ASN. Since the Mobile IP key is intended to encrypt signaling messages between the mobile terminal MS and the home agent HA, the authenticator does not need this Mobile IP key. Thus, in the traditional system, the access serving network ASN receives a key for which it has no need whatsoever, but which can nonetheless be used for manipulation purposes. For example, an unauthorized Mobile IP registration request message (MIPRRQ) could be sent by the gateway node of the access serving network ASN with the aid of the unnecessarily transmitted Mobile IP key. A further disadvantage is that the unnecessarily transmitted Mobile IP key needlessly occupies storage space in the ASN gateway node.